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Huge ice age methane blowout is ill omen for glacier retreat

The massive craters still seep methane

Andreia Plaza Faverola/CAGE

By Andy Coghlan

Call it the largest fart in Earth’s history. As the most recent ice age came to a close 12,000 years ago, retreating glaciers in the Barents Sea north of Norway triggered unprecedented blowouts of methane gas from massive dome-like features on the seabed.

Methane leaking from the permafrost today tends to be released very gradually or absorbed into vegetation or seawater. But evidence of the prehistoric blowouts, from dome structures on the seabed called pingos, suggests much larger and more rapid releases are possible.

With glaciers in Antarctica and Greenland retreating fast because of global warming, the big worry is that there could be pingos hiding underneath waiting to “go pop” in the same way, says Karin Andreassen at the Centre for Arctic Gas Hydrate, Environment and Climate at the Arctic University of Norway.

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Andreassen and her colleagues pieced together the story using detailed seismic and geologic data of the Barents seabed captured through high-resolution echo sounding from ships. They also looked at the composition of methane-containing gases still spewing up from the remains of 100 pingos.

Old methane

Analysis of the gas profiles revealed that the methane originated 30,000 years ago from hot zones of hydrocarbon-containing rock about a kilometre below the seabed. At that time, thick layers of ice over the seabed kept any methane that percolated upwards trapped in the uppermost layer of sediment down to depths of around 400 metres. Inside the sediment, the gas was converted into a solid ice-like mixture called a gas hydrate.

Then, 17,000 to 15,000 years ago, the ice sheet began to retreat rapidly, reducing the overhead pressure and causing hydrates to decompose. As they broke down, they released methane gas – the solid and stable gas hydrate layer became thinner and weaker. The pressure of the gas was strong enough to push up the seafloor to form pingo domes as much as a kilometre across.

By 12,000 years ago, the ice sheets had melted and the pingos were covered by an influx of warmer seawater from the Atlantic Ocean instead of ice. This made the hydrate layer so thin – just 30 metres – that the pingos could no longer withstand the pressure from below. The pingos burst, releasing the powerful greenhouse gas into the atmosphere, a process that probably took a few weeks per pingo.

The risks of something similar happening today are hard to predict, says Andreassen. The distribution of pingos on the seabed isn’t well charted elsewhere in the world. “We don’t have much seafloor data on pingo formation offshore of contemporary ice sheets like Antarctica and Greenland,” she says. But both regions are reckoned to be sitting on rich reserves of hydrocarbons.

Ice interplay

“Increased warming would increase the risk of releasing gas from gas hydrate pingos,” she says. But it would depend on the interplay of ice sheets, gas hydrates and permafrost in each area, as well as the capacity for gas to percolate upwards from much deeper hydrocarbon reserves. “It seems very unlikely that gas would be released from many areas simultaneously,” she says.

Other researchers are equally sanguine. “Unless a glacier happens to cap a deep thermogenic [hydrocarbon] source, the lack of any carbon would preclude the formation of massive shallow hydrates,” says Patrick Crill at Stockholm University in Sweden. “So I’m not too worried.”

Crill also points to previous research suggesting that over the past 800,000 years – including eight ice ages – atmospheric methane levels have varied between 350 and 700 parts per billion, much less than today’s 1900 parts per billion.

“There are spikes in the data, but they never exceed 700ppb, and to my mind, that represents all the inputs into the natural system prior to the Anthropocene,” he says.

In other words, the blowouts 12,000 years ago might have been large, but they appear to have had little impact on global methane levels at the time, which still fell well short of today’s levels.